Willoughby specifically researches the effects of training and nutritional intervention on the physiological, biochemical and molecular mechanisms regulating skeletal muscle strength, hypertrophy and atrophy in athletes. He has published articles on his findings in Journal of Nutrition and Metabolism, International Journal of Sport Nutrition and Exercise Metabolism and Journal of Medicinal Food.

Willoughby is a certified sport nutritionist, exercise physiologist, nutrition specialist and strength and conditioning specialist. He is a fellow of the International Society of Sports Nutrition, the American Society of Exercise Physiologists and the American College of Sports Medicine. He has been on the Baylor faculty since 2002.

After receiving his bachelor's degree in health and fitness from Tarleton State University in 1986, he earned his master of education degree there in 1989. He earned his doctoral degree in neuromuscular exercise physiology and biochemistry from Texas A&M University in 1993.